Slew: play? If its going you will hear grinding, popping or clicking noises. Check the grease on the bearings. If its got metal filings in it then its been grinding. (Declan#8)

Check for clean grease around nipples, all the pins should look like they have clean grease on them. Over-greasing is good!

Check it will hold the arm extended/bucket raised for a length of time, if hydraulics are on the way out then the arm gradually lowers. (proDave #3)

If you have a choice, get one with the main boom ram mounted on top of the arm - the older ones that have it below the arm are prone to damage and bending when something gets caught between the bucket and the ram. (PeterW#3)

Fully extend Spool valves and Rams - you may need an assistant to push the levers as far out/back as possible and look for weeps on the joints and also on the ram end seals. (PeterW#3)

Equipment: What does it come with? Quick hitch? Right buckets for your needs (Ed #9)

I don't agree with hire yard being in better condition. Operated only perhaps but self drive get battered. Sure they get fixed if they break. That's about it though. If you see an owner operator machine for sale they are much cleaner and seem to fetch a premium.